The present study was performed to elucidate the effect of interleukin (IL)-6 on the human GH (hGH)-gene expression in GH3 rat pituitary tumor cells using stable transfection of the hGH promoter fused to a luciferase reporter gene. Our results showed that IL-6 (10(2)-10(4) U/ml) stimulated GH secretion and synthesis, and promoted the luciferase expression in stably transfected GH3 cells with the maximal action of 1.99 times above the control by 10(4) U/ml IL-6. Among the inhibitors of signaling transduction pathways, mitogen-activated protein kinase kinase (MAPKK/MEK) inhibitor PD98059 (40 microM) and p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580 (5 microM) completely blocked the stimulatory effect of IL-6. Western blot analysis demonstrated that IL-6 indeed increased the activation of phosphorylated MEK and p38 MAPK in GH3 cells. Neither overexpression of Pit-1 nor inhibiting Pit-1 expression affected IL-6 induction of hGH-promoter activity. To identify the DNA sequence that mediated the effect of IL-6, six deletion constructs of hGH promoter were created. The stimulatory effect of IL-6 was abolished following deletion of the -196 to -132 bp fragment. In conclusion, our data show that IL-6 promotes GH secretion and synthesis by rat pituitary GH3 cells. The stimulatory effect of IL-6 on hGH-gene promoter appears to require the activation of MEK and p38 MAPK, and a fragment of promoter sequence that spans the -196 to -132 bp of the gene, but may be unrelated to Pit-1 protein.